Method of stabilizing xylidene



Patented May 30, 1950 "UNITED STATES NT -FHCE METHOD OF STABILIZING XYLIDENE poration of Delaware No Drawing. Application December 11, 1945,

Serial No. 634,404

4 Glaims. (01. 260-578) This invention relates to the stabilization of aromatic amines during storage, either alone or when used as an anti-knock addition agent in a motor fuel.

Aromatic amines such as aniline, toluidine, xylidine, etc. are normally substantially colorless when freshly prepared or freshly distilled, but tend to darken in color during storage, the undesirable result being greatly accelerated by storage at elevated temperatures.

It is also known that motor fuels, especially cracked gasoline and synthetic hydrocarbon fuel base stocks made by blending aroma-tics and acidtreated cracked stocks with alkylates, hydropolymers, aromatic blending agents, etc, tend to deteriorate during storage with resultant discoloration and gum formation. However, during the past fifteen years or so, a wide variety of different types of chemical compounds has been studied for use as gum inhibitors in such fuels, and it has been found that many materials coming within two relatively large classes of chemical compounds, namely alkylated phenols and amino phenols, have an oxidation inhibiting effect which serves to prevent such gum formation in motor fuels. On the other hand, neither of these two classes of compounds is of any value in stabilizing aromatic amines against discoloration and deterioration during storage, either alone, or in the presence of a major proportion of gasoline base stock; in fact, amino phenols such as normal butyl p-amino phenol, etc., generally accelerate the deterioration of the aromatic amines such as xylidine. A few compounds have been found which have fairly good stabilizing effect on the aromatic amines but these are not as satisfactory for some purposes as might be desired.

One object of the present invention is to provide a different class of stabilizer for the aromatic amines when used alone, and also to provide a more satisfactory stabilized motor fuel composition.

Broadly, the invention comprises adding to the aromatic amine a small amount of a non-aromatic amine. This stabilizer is a saturated organic amine and is preferably selected from the class consisting of cycloaliphatic amines, alkyl amines, and alkylol amines. In all three of these groups of materials, the amines may be primary, secondary or tertiary. For instance they may all be represented by the general formula RnNH3-n, where R. represents a cycloaliphatic, an alkyl, or an alkylol group, and n represents 1 to 3. Thus a primary amine coming within the general formula would have the subgeneric formula R-NH2, while a secondary amine'would have the formula RzNH, and a tertiary amine would have the formula :RsN. In the case of secondary and tertiary amines, the several R groups may be like or unlike and may be selected from. any one of the three above mentioned types of compounds or from two or more types. Instead of using pure compounds, one may use commercial products consisting essentially of the desired amines ormixtures thereof, either together with minor amounts of harmless impurities, or even containing a major proportion of substantially inert substance, which may for instance serve as a solvent.

Specific examples of suitable cycloaliphatic amines for use instabilizing aromatic amines, include cyclohexylamine having the formula Colin-NR2 methylcyclohexylamine CHaCcI-ImNHz, dimethylcyclohexylamine .(CHs)2CsH9NH2, ethylcyclohexylamine C2H5C6II10NH2, amylcyclohexyh amine C5H1lC6H10-NH2, dicyclohexylamine' (CsH11)2-NH, etc.

Among the alkyl amines which may be used as stabilizers, it is preferred to use those containing more than 5 carbon atoms per molecule, such as triethylamine (flzHs) 3N, diamylamine (C5H11)2NI-I,diblltylamil'le (Cd-IohNI-I, dipropyl amine (CsHflzNH, monobutyl monoamyl amine C4H9(C5I-I1'1)NH, isooctyl (di isobutyll amine CBHIINHZ, etc.

Specific examples of suitable alkylol amines, include monoethanolamine HOC2H4-NH2, diethanolamine (HOC2H4)2NH, etc.

Mixed saturated organic amines may be used such as diamyl-monocyclohexylamine (C5H1 )2NCsI-I1 1, dian yl monoethan o l :a m in e (C5H11)2NC2H4OH, etc.

In the various above described non-aromatic amines containing aliphatic groups, these allphatic groups may be either straight chained or branch chained and they may be attached to the amino nitrogen atom either through a primary (i. e. terminal) carbon atom or through a secondary or tertiary carbon atom (i. e. one which is in turn connected to two or three carbon atoms respectively) Although the invention is intended to apply broadly to the stabilization of any type of aromatic amines, including aniline and alpha or beta naphthylamine, it is believed particularly applicable to alkarylamines, especially those having from 1 to 3 alkyl carbon atoms. The invention has been found very successful for the stabilization of xylidine, either the pure separate isomers or mixtures thereof or relatively crude or partially or highly refined commercial products containing the same, as for instance a commercial xylidine product made by hydrogenation of nitroxylene. Xylidine has the general chemical formula (CH3) 2CsH3NH2. The two methyl groups in these xylidines may have various positions around the ring, in relation to the position of the amino group,'e. g., 2,3-; 3,4-; 2,6-; 2,4-; or 2,5-. Other alkarylamines which may be stabilized include the toluidines, ortho, meta or para, or mixtures thereof, the several ethyl anilines, cumidine (which is isopropyl aniline), etc. Dialkylated or trialkylated arylamines are preferred. Although the invention is particularly applicable to such alkarylamines, it ma also be used for stabilizing aniline, naphthylamine, and various derivatives thereof with either or both of the hydrogens of the amino group substituted by hydrocarbons, i. e. either alkyl or aryl groups, e. g. N-mono methyl aniline, N-dimethyl aniline, N-monoethyl aniline, N-dimethyl toluidine, N-dirnethyl xylidine, N-monomethyl xylidine, etc. Thus although the invention is particularl applicable to alkaryl primary amines, other amines such as secondary and tertiary may also be used. Also, alkaryl primary and secondary diamines may be used, such as N,N-dibuty1 paraphenylene-diamine, etc. The invention applies to aromatic amines made by any process, but particularly to those made by hydrogenation-reduction from the corresponding nitro aromatic compounds b the use of finely divided iron and hydrochloric acid.

The various above described aromatic amines, after being stabilized by having added thereto a small amount of a non-aromatic amine as described above, may be used alone for example as a dye intermediate, where discoloration of the aromatic amines is particularly objectionable, or they may be used together with a diluent such as a hydrocarbon solvent, e. g. a naphtha or a gasoline for use as a motor fuel. The alkaryl amines, e. g. xylidine, are particularly valuable for their anti-knock properties in motor fuels, especially for aviation engines, either with or without a metallo-organic anti-knock agent such as the various lead alkyls e. g. tetraethyl lead, tetramethyl lead, dimethyl diethyl lead, etc. The stabilized aromatic amines may also be diluted with alcohols, isopropyl ether, or other organic solvents or high octane blending agents to form the mixtures suitable for de-icing and knock suppression in aircraft engines. Such hydrocarbon, alcohol, or other organic diluent may be present in amounts ranging from only a few per cent up to 10, 20 or even 100 times the volume of the aromatic amine.

The amount of the stabilizer to be used will vary according to the intended duration of storage and the temperature of storage, as Well as according to the particular aromatic amine being stabilized and the particular stabilizer used, and will also depend upon whether the aromatic amines are to be stored alone or whether they are first to be added to a substantially larger volume of some diluting liquid such as the gasoline motor fuel base stock and then the resulting blend stored. Generally, however, if the aromatic amines are to be stored alone, or substantially alone, i. e. with less than one volume of diluent per volume of aromatic amines, then the amount of stabilizer to be used should be about l% to 0.5% by weight, and usually about 0.05% to 0.3% by weight of stabilizer will be best for commercial stabilization purposes. On the other hand, if the aromatic amines are added in a concentration of 0.1% to 3% or even up to 5% by volume to a gasoline base stock or other diluting liquid, the amount of the non-aromatic amine stabilizer to be used should be about 0.01% to 0.5% by weight, preferably about 0.05% to 0.3% by weight, on the amount of aromatic amine present, or about 0.0002% to 0.01%, preferably about 0.001% to 0.006%, by Weight based on the total motor fuel or other blended composition. Intermediate amounts of stabilizer may be required for instance for stabilizing an aromatic amine anti-knock agent concentrate in gasoline, for instance a 20% by volume solution of xylidine in a refined gasoline base stock, which may then later be added in desired proportion to a larger amount of motor fuel base stock to make a finished motor fuel blend containing xylidine in the desired proportion of for instance about 0.5% to 2%.

In carrying out the invention, it is preferred to add the stabilizer to the aromatic amine as soon as the latter has been prepared, e. g. reduced from the corresponding aromatic nitro compound, or immediately after distillation, the latter being preferably carried out under an inert atmosphere such as nitrogen so that a water-white distillate is obtained for stabilization.

The objects, advantages, and details of the invention will be better understood from a consideration of the following examples and experimental data.

Although amino phenol type inhibitors, such as n-butyl para amino phenol, are accepted for use in normal aviation gasoline containing no xylidine, they have proven detrimental to the color of xylidine and to the color of aviation gasoline blended with xylidine, as indicated by the following tests, where the amount of color degradation is indicated by the relative reduction from 100% transmission of light of 550 millimi-cron wavelength, before the test, to the various percentages indicated after the completion of 4 hours heating at 158 F. or after 6 months storage in a hot desert climate (approximate average temperature about to F. or higher) mittance at 550 Inhibitor Added (0.2+by wt.) mmu after 4 Hours xylidine Xylidine+inhibitor A Xylidine+inhibitor B Xylidine-l-iuhibitor C Color after 6 Months Storage in Per Cent Trans- Desert-Per Cent Transmittance at 550 mmu Base Fuel+0.0032% 1 inhibitor B 90. 9 Base Fucl+2% xy1idine+0.0032% 1 inhibitor 0. 31.3

The active ingredient in inhibitor A" B and C is n-butyl p-amino phenol, the essential diiference between the two being the nature of the alcoholic solvent present.

The above test-s indicate clearly that inhibitors A, B and C are actually detrimentalin that they accelerate *the color degradation of the Xy-lidine per se in an accelerated storage test, as well as when present in an aviation gasoline base stock, consisting essentially of a blend of virgin naph- 'tha, acid-treated catalytically cracked naphtha, hydropolymer, alky-late blending agent, isopentane, aromatic blending stocks and superfractionated isohexane.

In making the above described tests, the per cent light transmittance was determined by use of Coleman UniversalSpectrophotometer, model 11, according tostandard procedure.

Other amino phenol type inhibitors which are known to bedetrimental to the color of xylidine or to aviation fuels containing xylidine, are isobutyl p-amino phenol and the phenylene diamines.

For some unknown reason, a different clas of well known inhibitors for gasoline, i. e. the alkyla'ted phenol inhibitors, have been found com- Time for Reduct i o n t o 1 0% Trensmittance at 550 mu hours at 158 F.

Technical Xylid'me -e 46 Technical+0.2 wt. percent inhibitor D 43 Technical-+02 wt percent inhibitor E 40 Technical+0.2 Wt. percent inhibitor F 38 These tests show that whereas xylidine itself required 46 hours for a color degradation from 100% to transmittance, the use of 0.2% of various alkylated phenol inhibitors did not substantially change that time. The active ingredients in these three inhibitors respectively are: in inhibitor D an alkyl phenol (detailed structure not known), in inhibitor E 2,4 dimethyl G-tertiary butyl phenol, and in inhibitor F a reaction product of p-cresol with isobutene believed to be a 2,6-di-tertiary butyl p-cresol.

Other alkylated phenols which are known to be satisfactory for inhibiting gum formation in aviation gasoline, and which are either known or believed to be without serious detrimental effeet on the color of xylidine, are the alkyl phenols derived from petroleum.

Example I In contrast to the various tests listed above, it has been found according to the present invention, that non-aromatic amines, especially saturated cycloaliphatic amines, alkyl amines and alkylol amines, are far superior to the amino phenols and alkylated phenols for use in arcmatic amines either alone or in the presence of a motor fuel base stock, because these saturated amines not only are not detrimental to the color of the aromatic amines but actually have a positive and beneficial effect in preventing the color degradation of the aromatic amines during stor- .Xylidine 6 age. The following table shows the favorable results obtained with 0.2% by weight of dimethyl cyc-lohexylamine in some commercial xylidine made by reducingnitro-Xylene by finely divided iron and hydrochloric acid, 'as 'compared to *the same concentration of inhibitor D (referred to above) and compared to a sample of the same xylidine without any inhibitor or stabilizer.

Time 'for Reduc- Ltai on to 10% 'lr'ansmittance zit/ 550 mmuliours atCl58 F.

Xylicliue-PO;

These tests show that 0.2% 'by weight of dimethyl cyclohexylamine more than doubles the storage stability of the particular sample of "Xylidine being tested, and that under 'thesame conditions inhibitor D has no stabilizing effect whatsoever.

Example II Another series of tests was similar to those of Example I except that a di fierent sample of commercial xyl-idine was used, and in this case three different non aromatic amines were used as stabilizers, with the following results:

Time for Reduction to 10% Transmittance at 550 mmu hours at l58 F.

Xylirline 19 Xylidine+Triethylarnine 50 Xylidine+ Cyclohexylamine. Xylidine-l-Mono-ethanolamiue Example III To show that the non-aromatic amine used for stabilizing the Xylidine, and the alkylated .phenol used for stabilizing the gasoline base stock, are compatible with each other, several blends were prepared and tested for color deterioration after subjection to the Modified Army Gum Oxidation test with the following results:

Average Overall Per cent Transmittance (400-600 mmu) after Modified Army Gum Oxidation, Expressed as Per cent of the Transmittance of the Base Fuel after Same Oxidation Proceduro Gasolinc+l% Xylidinc r. Gasoline-l-inhibitor D -.e Gasolinc+inhibitor+ o ylcyclohexylaminc These tests show that when fresh (unstored) Xylidine is added to a gasoline base stock, an aikylated phenol such as inhibitor D, referred to previously, which is useful for stabilizing the gasoline against gum formation, is compatible with both the gasoline'base stock and the xylidine in that it does not accelerate color deterioration, and in fact even makes a slight improvement in the color, and that the further addition of dimethylcyclohexylamine to the blend in order to stabilize the xylidine during storage, does not alter the compatibility of the several ingredients of the final blended composition, as indicated by a color test after the Modified Army Gum Oxidation test. It should be understood of course, that fresh Xylidine added to a gasoline containing alkylated phenol inhibitor would be stable without dimethylcyclohexylamine, but Xylidine which had been stored for a time would require the dimethylcyclohexylamine throughout the storage period to yield a stable fuel when added to a gasoline containing an alkylated phenol. The mixture prepared by blending Xylidine plus dirnethylcyclohexylamine stabilizer with a gasoline to which an alkyl phenol inhibitor had been added would therefore possess permanent properties, i. e. storage stability, superior to those of a fuel containing only Xylidine, gasoline, and the alkylated phenol.

It is not intended that this invention be limited to the particular materials, which have been re cited merely for the sake of illustration, but only by the appended claims in which it is intended to claim all novelty inherent in the invention as well as all modifications coming within the scope and spirit of the invention.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,947,578 Bond Feb. 20, 1934 2,110,199 Carothers Aug. 3, 1934 2,180,427 Moncrief et al. Nov. 21, 1939 2,322,572 Fisher June 22, 1943 2,215,038 Hodgins Sept. 17, 1943 2,361,337 Walters Oct. 24, 1944 2,410,847 Walters Nov. 12, 1946 2,422,484 Herbst June 17, 1947 2,461,917

Oreluqo Feb. 15, 1949 

1. METHOD OF STABILIZING XYLIDENE AGAINST DISCOLORATION DURING STORAGE, WHICH COMPRISES ADDING TO THE XYLIDENE 0.01 TO 0.5% BY WEIGHT OF A MONOCYCLOHEXYLAMINE. 